1. Field
The present invention relates to the field of optical fibers.
More specifically, the present invention relates to the design of an optical fiber suitable for producing energy-dissipative and wavelength-selective filters capable of acting as doped-fiber amplifier gain equalizing filters.
2. Description of the Related Art
The field of optical fibers has already given rise to a vast amount of literature.
An optical fiber is a waveguide. It is called a monomode fiber when only a single light mode propagates through it. This mode is called the fundamental mode.
Those skilled in the art know that certain dopants (germanium, fluorine, etc.) inserted into the fiber are sensitive to light radiation (ultraviolet radiation in the case of the dopants mentioned) and that the physicochemical reactions caused by this radiation may modify the refractive index of the photosensitive part.
A periodic perturbation of the refractive index of the photosensitive part of the fiber constitutes a Bragg grating.
The spectral response of short-pitch Bragg gratings produced in an optical fiber results from the coupling of the copropagative fundamental mode into counterpropagative cladding modes and into the guided coupling corresponds to the part which is reflected in the firection opposite to that of the fundamental mode (it is also called reflection or reflected power). The counterpropagative cladding modes are modes propagating in the cladding which surrounds the core of the fiber. If the grating is coated with a material whose index is close to that of silica, the cladding modes then become radiative modes. All the latter leak to the outside of the fiber (See, V. Mirahi and J. E. Sipe, xe2x80x9cOptical properties of photosensitive fiber phase gratingsxe2x80x9d, IEEE, Journal of Lightwave Technology, Vol. 11, No. 10, October 1993; E. Erdogan, xe2x80x9cFiber Grating Spectraxe2x80x9d, IEEE, J.L. Technology, Vol. 15 No. 8, 1997).
Moreover, those skilled in the art know that the fact of tilting the lines of the grating with respect to the axis of the fiber promotes coupling into the radiative modes to the detriment of guided counterpropagative coupling.
Such gratings have already been produced in a fiber having a photosensitive core in order to make the gain of doped-fiber amplifiers uniform. the results are presented in reference (See, R. Kashyap, R. Wyatt and R. J. Campbell, xe2x80x9cWideband gain flattened erbium fibre amplifier using a photosensitive fibre blazed gratingxe2x80x9d, Elect. Lett., Vol. 29, pp 154-165, 1993). An amplified spontaneous emission (ASE) spectrum has been equalized to xc2x10.5 dB at 35 nm. The reflected power problem is very briefly discussed. In reference, Erdogan shows that the change in the power reflected by the grating as a function of the angle of tilt of the lines is oscillatory (E. Erdogan, J. E. Sipe, xe2x80x9cTilted fiber phase gratingsxe2x80x9d, J. Opt. Soc. Am. A, Vol. 13, No. 2, 1996). He has shown that the amplitude of these oscillations is smaller in the case of the writing of gratings into a fiber in which part of the cladding is as photosensitive as the core (see, L. Brilland, D. Pureur, J. F. Baton and E. Delevaque, xe2x80x9cSlanted gratings UV-written in photosensitive cladding fibrexe2x80x9d, Elect. Letters, Vol. 35, No. 3, pp 234-236, 1999). Tilted gratings written into a fiber with a photosensitive cladding have already been used to equalize the 1 543.5 nm to 1 561.5 nm erbium gain band (see, I. Riant, L. Gasca, P. Sansonetti, G. Bourret, J. Chesnot, xe2x80x9cGain equalization with optimized slanted Bragg grating on adapted fibre for multichannel long-haul submarine transmissionxe2x80x9d, Optical Fiber Communication Conference, ThJ6, San Diego, February 1999).
However, as far as the inventors are aware, despite the extensive research conducted in the field, no gain equalizing filter composed of tilted Bragg gratings produced in a fiber with a photosensitive core and photosensitive cladding has yet been developed in order to make the gain of fiber amplifiers uniform over a wavelength range of more than 20 nm.
The objective of the invention is to improve the known state of the art by proposing such a device.
This objective is achieved within the context of the present invention by means of an optical fiber whose core and part of the cladding are photosensitive to radiation, for example ultraviolet radiation, and in which fiber the difference in refractive index (xcex94n) between the core and the cladding is less than 3xc3x9710xe2x88x923.
According to another advantageous feature of the present invention, the normalized frequency parameter V is less than 1.5 at a wavelength of 1.55 xcexcm.
According to another advantageous feature of the present invention, the radius b of the photosensitive cladding is at least 1.5 times greater than the radius of the core a.
According to another advantageous feature of the present invention, the tilt of the lines of the Bragg grating with respect to the axis 0xe2x80x940 of the fiber is great than 3xc2x0.
According to another advantageous feature of the present invention, the fiber comprises three to five Bragg gratings having different spectral characteristics.